We have measured regional cerebral blood flow (rCBF) with oxygen-15 and positron emission tomography in normal subjects during performance of a battery of frontal lobe tasks putatively involving working memory as well as during performance of matched sensorimotor control tasks for each frontal lobe task. The results indicate that tasks involving working memory activate a network including dorsolateral prefrontal cortex (particularly the inferior frontal gyrus in slices falling between six to eight centimeters above the canthomeatal line) and superior parietal cortex (particularly the inferior parietal lobule and area 7) in humans. During more complex tasks hippocampus is also activated. Studies of normal monozygotic twins aimed at determining the degree of heritability of cognitively-related regional brain function suggest that while there are more similarities between twins than between age and sex- matched unrelated individuals, the degree of genetic influence may not be large.